The present invention relates generally to the cooling of multilevel entities such as atoms, ions or molecules, and in particular to the cooling of such entities by promoting coherent scattering of light from them with the aid of a resonator.
The question how to efficiently cool atoms or how to reduce their kinetic energy arises in many circumstances, including situations where accurate measurements of atomic energy levels are required. For example, an atomic clock based on the Ramsey method requires knowledge of the energy levels (transition frequencies) of cesium atoms for calibration purposes. The cesium atoms have to be moving slowly to yield sufficiently accurate energy level measurements. A good method to achieve this result is to cool the cesium atoms. In numerous other applications, atoms have to be confined within small volumes and a convenient technique of accomplishing this goal is to reduce their kinetic energy through cooling.
An effective way to cool atoms with the aid of electromagnetic radiation was developed in the 1980""s by Steven Chu and is described in S. Chu et al., Physical Review Letters, Vol. 55, pp. 48-51, (1985). Under most circumstances irradiating atoms is with electromagnetic radiation will cause heating. Under special conditions, however, it is possible to use pairs of laser beams properly positioned and operated to reduce atomic motion. Upon this discovery laser cooling of cesium atoms was adapted in atomic clocks as described, for example, in U.S. Pat. Nos. 5,338,930; 5,528,028 awarded to Chu et al. In fact, laser cooling has also been a tremendously successful technique for creating high-brightness atomic sources for various other applications, as proposed by T. W. Hxc3xa4nsch and A. L. Schawlow, Optics Communications, Vol. 13, p. 68, (1975).
Early laser cooling experiments were performed inside atom traps, such as magnetic bottles. The atoms were cooled when they encountered a laser beam containing photons coming at them with an energy that was less by an amount xcex94 than the energy that would normally be absorbed if the atoms were stationary. In fact, the moving atom can absorb lower energy photons than the stationary atom as long as the detuning xcex94 compensates for the Doppler effect of the moving atom. Later, the atom emits a photon whose frequency is equal to the energy of the absorbed photon plus the Doppler effect v/xcex, where v is the atom""s velocity and xcex is the wavelength of the light. In other words, the emitted photon has a higher energy than the energy of the absorbed photon by the amount of Doppler effect, which is also the amount of kinetic energy the atom loses in the process.
On the heels of the above discovery, the general principle of absorbing lower energy photons and emitting higher energy photons to carry away kinetic energy and achieve cooling has been studied in more detail. For example, in U.S. Pat. No. 5,615,558 Cornell et al. teach a device and method for laser cooling of a solid to extremely low temperature using a high purity surface passivated direct band gap semiconductor crystal. The crystal is cooled when illuminated by a laser beam. Cooling is caused by emission of photons of higher energy than photons entering the crystal, the additional energy being accounted for by absorption of thermal phonons from the crystal lattice.
The prior art also teaches a fluorescent refrigerator in which a working material absorbs substantially monochromatic electromagnetic radiation at one frequency and then emits fluorescent radiation that has, on the average, a higher frequency. More energy is thereby removed from the working material than is introduced into the material, the difference between the output energy flux and the input energy flux being supplied by the thermal energy of the working material. More recent laboratory measurements have demonstrated laser-induced optical refrigeration in solids and liquids, see, e.g., C. E. Mungan et al., Physical Review Letters, 78, pp. 1030-1033 (1997) and J. L. Clark and G. Rumbles, Physical Review Letters, 76, pp. 2037-2040, (1996). More information on fluorescent refrigeration can also be found in U.S. Pat. No. 5,447,032 to Epstein et al. and R. I. Epstein et al., Nature, 377, pp. 500 (1995).
In U.S. Pat. No. 6,041,610 Edwards et al. teach improvements to an optical refrigerator operating on the above-described principles by using reflectivity-tuned dielectric mirrors. Again, the working materials are pumped using monochromatic radiation such that the resulting fluorescence has an average photon energy higher than that of the pumping radiation. The parallel-mirrored faces of the mirrors are employed to increase the optical path of the incident pumping radiation within the working material by multiple reflections. The mirrors are chosen to allow the higher-energy fluorescence photons to escape from the working material to carry away thermal energy while inhibiting the escape of the lower-energy photons that are consequentially partially trapped in the working material and ultimately reabsorbed to promote further fluorescence. This approach of extending the optical path length of the lower energy photons and minimizing the path length of higher energy fluorescence photons increases the optical refrigerator efficiency.
The prior art also addresses alternative ways of trapping atoms for cooling. T. W. Mossberg et al. describe in xe2x80x9cTrapping and Cooling of Atoms in a Vacuum Perturbed in a Frequency-Dependent Mannerxe2x80x9d, Physical Review Letters, Vol. 67, No. 13, pp. 1723-6 (Sep. 23, 1991) how trapping atoms in colored vacua can achieve large capture velocities and capability to cool to temperatures well below the Doppler limit present in free-space cooling techniques. For example, colored vacua can be achieved in suitably designed resonant cavities, and, according to the findings of T. W. Mossberg et al., can dramatically enhance the effect of transfer of kinetic energy of two-level atoms into the electromagnetic-field energy (also referred to as a Sisyphus-type effect).
Further possibilities for all optical trapping and cooling of two-level atoms are explored by P. Horak et al. in xe2x80x9cCavity-Induced Atom Cooling in the Strong Coupling Regimexe2x80x9d, Physical Review Letters, Vol. 79, No. 25, pp. 4974-4977 (Dec. 22, 1997). The authors concentrate on trapping and cooling a single atom at the antinodes of a high Q cavity mode to which the atom is strongly coupled.
All of the above-discussed methods of cooling atoms and materials have a number of shortcomings. The methods for cooling atoms in free space with suitable laser beams are limited by the Doppler recoil limit, at which on-coming photons will no longer be absorbed and the atom that is to be cooled recoils. In addition, all of the above-mentioned techniques can only be used in cooling two-level atoms that have a well-defined internal structure, i.e., a dominant two-level absorptive transition. That is because the detuning energy xcex94 has to be specifically selected with the two-level transition in mind, as the atom has to absorb many photons with this detuning energy xcex94 to experience appreciable cooling. These criteria severely limit the types of atoms that can be cooled by the prior art techniques.
Most atoms and all molecules have multiple ground states to which the excited state can decay. Once the atom reaches a different ground state, the laser no longer has the correct detuning relative to the atomic transition, and the cooling stops. In particular, molecules have many vibrational and rotational levels, and consequently no laser cooling of molecules has been demonstrated. If we could learn how to cool, trap and manipulate larger molecules in the same way as atoms, this would open the door for important developments in chemistry and biology.
In view of the shortcomings of the prior art, it is a primary object of the present invention to provide a method and apparatus for efficiently cooling multilevel entities including atoms, ions and molecules, as well as entities without an internal level structure in the optical domain, including electrons and protons. More particularly, the invention is intended to provide for cooling such multilevel entities below the Doppler limit.
It is another object of the invention to provide a cooling technique that can be implemented in a straightforward manner in well-known types of optical cavities.
These and numerous other advantages of the present invention will become apparent upon reading the following description.
In accordance with the present invention multilevel entities such as atoms, ions or molecules are cooled by coherent scattering, where the frequency of the emitted radiation exceeds the frequency of the illumination radiation. Such coherent scattering is achieved in a resonator, typically a cavity provided for containing the multilevel entities. The cavity length and mirror coating are selected to support a resonant radiation. The multilevel entities are illuminated with an illumination radiation whose energy is lower than that of the resonant radiation supported by the resonator. Specifically, the energy of the illumination radiation is lower by a certain detuning energy from that of the resonant radiation. The detuning energy is selected such that coherent scattering of resonant radiation from the multilevel entities at a higher frequency than that of the illumination radiation is promoted by the resonator. As a result of the coherent scattering energy is carried away from the entities and they are cooled. Since the detuning energy is selected with respect to the energy of the resonant radiation supported by the cavity, rather than any specific atomic or molecular transition, the method of the invention can be used to cool the center-of-mass motion of various multilevel entities, including atoms, ions and molecules without concern for the energy levels of the multilevel entities. The entities can be present in the form of a gas, a solid or a liquid. The method can also be used to cool entities exhibiting no internal level structure at the frequencies of the illumination radiation (e.g., optical frequencies) including elementary particles such as electrons and protons.
In one embodiment of the invention the detuning energy is selected to correspond to an internal transition of at least one of the multilevel entities to further cool the internal degrees of freedom of that multilevel entity. The transition can be any energy transition, including a transition associated with a rotational and/or vibrational degree of freedom when the entity is a molecule. In cases where the multilevel entities are presented in the form of a solid, the detuning energy can be selected to correspond to an internal transition associated with a phonon.
In most embodiments, the illumination radiation is injected into the cavity, e.g., through the cavity mirrors or from the side. Preferably, the illumination radiation is provided by a laser.
In a preferred embodiment, the resonant radiation in the cavity is amplified. Conveniently, the level of amplification is adjusted such that a single-pass gain experienced by the resonant radiation in the cavity partly compensates or, preferably exceeds round-trip reflection losses sustained at the cavity mirrors. Such adjustment can be made, e.g., by selecting an appropriate amplifying medium. In this case the cavity with amplifying medium acts as a cavity with much higher mirror reflectivity, which enhances the cooling force.
The invention can be practiced in a number of optical cavities. For example, a spherical cavity can be used in order to maximize the solid angle subtended by the cavity such that a large number of the scattered photons contribute to the resonant radiation. It is preferable, however, to use a confocal cavity because such cavity provides a large cooling volume within which the entities can be trapped and cooled.
A detailed description of the invention and the preferred and alternative embodiments is presented below in reference to the attached drawing figures.